Unit 4 - Materials

What is the density of a material?

Its mass per unit volume and its a measure of how compact a substance is

What does Hooke’s law state?

That extension is directly proportional to the force applied given that the environmental conditions are kept constant.

How can Hooke’s law be shown?

By the straight part of the force-extension graph. A straight line through the origin shows they are directly proportional

What is the limit of proportionality?

The point after which Hooke’slaw is no longer obeyed

What is the elastic limit?

Is just after the limit of proportionality where the material will deform plastically

What is the equation that corresponds with Hooke’s law?

F = kL

What is tensile stress?

Force applied per unit cross-sectional area

What is tensile strength?

What is caused by tensile stress and is defined as the extension over the original length

When is elastic strain energy stored?

When work is done on a material to stretch or compress it.

How can elastic strain energy be calculated?

By calculating the area under a force-extension graph = 0.5FL

What is breaking stress?

The value of stress at which the material will break apart which will depend on the conditions of the material

What are the behaviours that a material can exhibit on a force-extension graph?

Plastic and brittle

What does plastic mean?

Where a material will experience a large amount of extension as the load is increased especially beyond the elastic limit

What does brittle mean?

Where the material will extend very little and therefore is likely to break at a lower extension

What is the difference between loading and unloading beyond the elastic limit?

It has stretched too much so will not return to its original shape or the origin on the graph.

How do you calculate the work done of beyond the elastic limit?

The area between the loading line and the unloading line of a force extension graph

What happens to the work done when the stretch is elastic?

It is stored as elastic strain energy

What happens to the work done when a stretch is plastic?

It moves atoms apart and dissipates as heat

What is the difference between a stress-strain graph and a force-extension graph?

They describe the behaviour of a material rather than the behaviour of a specific object

What is a material’s ultimate tensile strength stress?

The highest point on the graph as it shows the maximum stress the material can withstand.

What does the shape of a stress-strain graph show?

Whether a material is ductile, plastic or brittle

What does ductile mean?

Can undergo a large amount of plastic deformation before fracturing

What is the young modulus?

It describes the stiffness of a material because stress is proportional to strain therefore the value of stress over strain is constant

What is the formula for young modulus?

Tensile stress / tensile strain

What is the gradient of a stress-strain graph?

Young modulus